lib: libc: arcmwdt: add declarations for reentrant time functions

ARC MWDT library does not contain implementations for the reentrant
POSIX time functions, so common Zephyr implementations end up being
used instead. However, there are no declarations, so we just add them
here, unguarded, until a better place is found.

Signed-off-by: Ilya Tagunov <Ilya.Tagunov@synopsys.com>